Conventional piezoelectric accelerometers include a piezoelectric element, made with piezoelectric crystals or ceramic materials, that responds to stimuli from mechanical stresses exerted on their surfaces, by a seismic, or inertial mass; in direct proportion to external physical phenomena, like acceleration, pressure, or force, and other environmental stressors. Piezoelectric accelerometers can operate at wide ranges of temperature, and usually are divided in two groups; as known; charge mode and voltage mode. Charge mode accelerometers are high impedance units, that can operate at very high temperatures, limited by the construction materials and the physical limitations of the piezoelectric element. A charge mode accelerometer can be made to operate at temperatures above 1000° F. A charge mode accelerometer requires an external charge amplifier and other ancillary instrumentation for signal conditioning. Voltage mode accelerometers are low impedance units, which have their advantages with the instrumentation to process the signals from the sensor. However, limited on the maximum temperature of operation, by the electronic parts inside the unit. Usually, the maximum temperature for a unit with internal electronics is 325° F.
Accelerometers intended for high temperature of operation, from 400° F. up to 1,600° F., use special piezoelectric crystals, or piezoelectric ceramic materials, that can operates at those temperatures. Usually, piezoelectric crystals and ceramics, like Lithium Niobate, Langatate, Gallium Orthophosphate, Bismuth Titanate, modified Lead Titanate Zirconates, Lead meta Niobates, and others; are the materials of choice for high temperature applications, and not only in accelerometers, but any piezoelectric sensor using these, like ultrasonic sensors, for NDT [non destructive test], and for pressure, or force sensors. Piezoelectric materials develop an electric charge upon stress on an specific orientation, and this can be used to relate to a physical input. The main property of these piezoelectric materials [used in high impedance sensors] is that they are highly dielectric materials, and they should be like that, because, if not, when you develop an electric charge, it will dissipate through the material when it's electrically conductive. Therefore, piezoelectric materials have, and require to have, a very high resistivity; and the electronics used to read a meaningful signal, also requires that the piezoelectric element is highly resistive. In general, the piezoelectric materials will start to increase their electrical conductivity with the increase of the temperature. Examples of resistivity varies, from the Mega Ohms range, to drop to the Kilo Ohms range. Basically, for certain operational temperature a manageable value of around 100 k Ohms can be used. Standard practice uses the piezoelectric materials up to a temperature, just half, or a maximum of two thirds of the Curie temperature for the piezoelectric material. Above the Curie temperature the piezoelectric properties are lost permanently.
The majority of the piezoelectric materials, single crystals or ceramics, are oxides. This means that they are metals, or non metals, combined with Oxygen and oxidized by it. Moreover, the oxides of metals, in general are highly dielectric, or non electrically conductive, as opposed with metals that usually are electrically conductive or semiconductors. The bonding of Oxygen with different elements, or metals, have different strengths and stabilities. Some are very strong bonds, and cannot be reduced easily, like Silicon Dioxide, or very electropositive elements, like Lithium or Sodium with Oxygen. However, certain transition elements (metals) can be reduced to zero valence, metallic state, from the oxide, just by applying heat. Some of the piezoelectric materials, even in single crystal form, or ceramic; can be reduced slightly (less than stoichiometric), losing Oxygen, and becoming more electrically conductive by the increase of temperature. This undesirable property is exacerbated, when the piezoelectric material is in a reducing atmosphere, vacuum, sealed container, or with low partial pressure of Oxygen. When the Oxygen released by the piezoelectric material is reacting with other internal metal components of the sensor, the term Oxygen depletion is applied, because usually, it is permanently lost. The process is reversible, in general, therefore, if a source of Oxygen is applied to the internal parts of the sensor; it can be recovered.
Solutions adopted by the industry using these piezoelectric materials at high temperatures and in sealed containers (housings) are the following:
One simple solution to the Oxygen depletion proposed by many inventors in the prior art, is to keep the accelerometer or piezoelectric sensor vented with an opening to allow Oxygen from Air, to replace any Oxygen losses inside the sensor. Unfortunately, there are applications that requires a sensor to be hermetic, and impervious to the external environment. Manufacturers hide the venting hole, to be located at a non splashing location, but moisture and reactive gases can enter the housing and produce corrosion, or other internal damage on the sensor, even when it is not in operation. Plus, if the sensor is used in a reducing atmosphere, it won't matter if the sensor is vented; but necessary to be hermetic. An example of this type of solution is shown and described in U.S. Pat. No. 3,727,084.
A second approach of having a vent, but not fully open, is to use a porous metal aggregate, which allows to exchange gases between the interior and the exterior of the sensor. Unfortunately, the same implications discussed above for a non hermetic sensor are applicable, therefore is not a real solution for the Oxygen depletion problem. Moreover, the porous metal plug can get clogged eventually, terminating the benefit of the sought property. An example, of this type of solution is shown and described in U.S. Pat. No. 5,209,125.
Other researchers added inside the sensor a chemical that can release Oxygen to increase the partial pressure of Oxygen inside the unit. An example of a chemical that will release Oxygen when heated is Manganese Dioxide, taught also by U.S. Pat. No. 3,727,084 If the unit is hermetic to a level better than 1×10−8 atmosphere*cc/sec, then the leakage is small enough to maintain the optimum performance of sensor for over 10 years, before the internal gases are exchanged with the external environment. Unfortunately, there are applications that cannot work properly with loose particles trapped inside the housing of the sensor. Accelerometers are design and made to work under vibration, and could release particles from the chemical itself, or the reactant left after the is Oxygen released. Same with ultrasonic probes, in which the high frequency of operation may disturb the layers of chemicals intended for oxygenation. Careful consideration during the design of the sensor may ameliorate the operational downsides.
Another technique to control the Oxygen depletion inside piezoelectric sensors is to backfill the housing with Oxygen, or enriched gases with Oxygen, or to have a path for Oxygen to interconnect the inside of the sensor housing in order to increase the partial pressure of Oxygen inside the sensor housing. Of course, again if the container or housing of the sensor is hermetic this will work. Unfortunately, the problem with this technique is that the sealing of housings in the sensor industry is typically made by laser welding or E-beam welding. Laser welding is performed at atmospheric pressure and can be backfill with different gases, but in pure Oxygen, or highly enrich environment of Oxygen will ignite the metal housing. E-Beam welding is performed under vacuum, and cannot be backfilled and sealed at the same time. Only a cold welding method will allow to backfill the sensor with Oxygen, however this technique will complicate the manufacturing process quite a bit. An example, of this type of solution is shown and described in U.S. Pat. No. 7,650,789.
Yet another technique to mitigate the Oxygen depletion of the piezoelectric materials is to pre-oxidize all internal components prior to seal. In theory, by pre-oxidizing all the internal parts of the sensor, they won't take Oxygen released by the piezoelectric materials. This is a factual statement, because if the materials from the housing or other components are been oxidized by Oxygen from the piezoelectric elements, then, this Oxygen will be loss permanently and the sensor not only, won't perform properly, but it will be damaged permanently. The pre-oxidation of the internal parts can be performed by actual oxidation of parts in Air at higher temperatures than normal operation, or by chemical passivation of the parts, both techniques have their merits and advantages. Unfortunately, this technique is necessary for the proper manufacturing of the sensors intended for high temperatures, but it won't correct the drop in resistivity on the piezoelectric elements, therefore, the sensor won't work correctly. This technique is not addressing the cause of the problem on the piezoelectric element, but one of the consequences. An example, of this type of solution is shown and described in U.S. Pat. Nos. 7,650,789 and 5,209,125.
Similarly, other researchers proposed to use noble metal liners on the inside of the housing of the sensor, in order to avoid taking free Oxygen released by the piezoelectric materials. Examples of this technique are the use of Platinum, Gold, and other metals that won't oxidize under the operational conditions of the sensor. Unfortunately, as with the previous technique, it won't address the issue of the loss of resistivity of the piezoelectric element.
In another proposed solution, researchers used diffusion barriers to avoid the release of Oxygen from the piezoelectric materials. There is some validity to this technique, but careful consideration of the chemistry of these barriers should be taken care also. The piezoelectric materials are highly dielectric and specific properties (electrical and mechanical) on them are sought, any diffusion barrier modifying these properties will be detrimental to the performance of the sensor. In some cases, a compromise of these properties can be obtained, then, this solution will be valid in those cases.
Some piezoelectric materials do not have the Oxygen depletion problem, like quartz, Gallium Orthophosphate, etc. However, quartz in the form of the most common form, alpha-quartz, can be use up to 500° F., when is clamped under stress inside the sensor, even when its phase transition (alpha-beta) occurs at 1063° F. Some researchers proposed to use beta-quartz, but this form is only stable over 1063° F., unless a high pressure. Unfortunately, quartz is a very poor conductor of heat, and sensors build with this material will be susceptible to thermal shock, which permanently damage the sensor. At these high temperatures quartz requires heating and cooling rates of around ten degrees per hour to avoid a destructive thermal shock. The material Gallium Orthophosphate is a new promising piezoelectric crystal, but it has the disadvantage of its very high cost, reduced availability with only two commercial vendors worldwide, and the material still not mature enough for the accelerometer and sensor market. As with quartz, this material will suffer with fast thermal shocks, plus the manufacturing technique is not well developed yet, to eliminate the excessive twining found on this material.
Besides working with the piezoelectric material issues at high temperature, there are other problems with the instrumentation required to obtain a meaningful signal from the piezoelectric sensors. A great deal of work was made in developing charge amplifiers that can take very low resistivity and still provide a proper response, corresponding to the external stimuli to the sensor. Unfortunately, the signal to noise ratio of any charge amplifier will be very low, when the resistivity of the piezoelectric material is changing a such larger magnitude (from ranges in the 107 Ohms to 103 Ohms). The signal will show more thermal noise, phase shifts, spikes, perturbations, etc.; than in normal conditions.
There are applications in superconductor material (metals oxides, similar to the piezoelectric materials) in which Oxygen stability on the compound matrix is the key factor in obtaining meaningful results at higher temperatures. Researchers in superconductors have to settle for lower temperature materials, due to fact, that Oxygen loses are less pronounced in these. An example, of this type of situation is shown and described in U.S. Pat. No. 5,972,845.
This invention provides an effective solution to overcome all the imperfections of the prior art.